Title: Sedimentology, sandstone provenance and palaeodrainage on the eastern Rockall Basin margin: evidence from the Pb isotopic composition of detrital K-feldspar
Abstract: The Rockall Basin, west of Ireland, is a frontier area for hydrocarbon exploration but currently the age and location of sand fairways through the basin are poorly known.A recently developed provenance approach based on in-situ Pb isotopic analysis of single K-feldspar grains by laser ablation multi-collector inductively-coupled mass spectrometry (LA-MC-ICPMS) offers advantages over other provenance techniques, particularly when applied to regional palaeodrainage issues.K-feldspar is a relatively common, usually first-cycle framework mineral in sandstones and its origin is typically linked to that of the quartz grains in arkosic and sub-arkosic rocks.Consequently, in contrast to other techniques, the Pb-in-K-feldspar tool characterises a significant proportion of the framework grains.New Pb isotopic data from K-feldspars in putative Permo-Triassic and Middle Jurassic sandstones in Well 12/2-1z (the Dooish gas condensate discovery) on the eastern margin of the Irish Rockall Basin are reported.These data suggest that three isotopically distinct basement sources supplied the bulk of the K-feldspar in the reservoir sandstones and that the relative contribution of these sources varied through time.Archaean and early Proterozoic rocks (including elements of the Lewisian Complex and its offshore equivalents), to the immediate east, north-east and north of the eastern Rockall Margin, are the likely sources.More distal sourcelands to the north-west cannot be ruled out but there was no significant input from southern sources, such as the Irish Massif.These data, together with previously published regional Pb isotopic data, highlight the important role played by old, near and far-field Archaean -Proterozoic basement highs in contributing sediment to NE Atlantic margin basins.The Irish Massif appears to have acted as a significant, but inert, drainage divide from the Permo-Triassic to the Late Jurassic and hence younger, Avalonian and Variscan, sand sources appear to have been less important on the Irish Atlantic Margin.(abstract ends).Understanding the provenance of sandstones helps in the reconstruction of palaeogeography and in constraining drainage scales and sediment pathways in sedimentary basins.This helps to predict the source and distribution of sandstones, with implications for reservoir quality in the subsurface.However, there are some inherent shortcomings in provenance analysis.Provenance studies are limited by the ability to geochemically distinguish the wide range of potential sourcelands.Additionally, their specific use in the reconstruction of robust palaeodrainage models is problematic, as there are difficulties in recognising and quantifying the extent of sedimentary recycling and mixing (Tyrrell et al. in press).